Transistorized amplifier



Sept 24 1963 o. HELLER' TRNSISTORIZEID AMPLIFIER Filed June 5, 1959United States Patent O 3,165,399 TRANSSTQPZEB AMPLEFR @scar Heller, Newl rit, NIX., assigner to Bulova Re- Search and Development Lahoratories,lne., Woodside, NX., a corporation of New Yori;

Filed .lune 5, i959, Ser. No. 313,3?? 3 Claims. {(Il. 33h-l5) Thisinvention relates generally to electronic -ampliiiers `and moreparticularly to `'an improved traneistorized amplifier adapted tooperate efficiently under adverse environmental conditions.

The expanding applications for electronic equipment are such thatamplifiers must now be incorporated into devices which are subiected toa wide range of temperature variation and which undergo a high order ofshock acceleration and vibration. Such conditions are met, for example,in the electronic control systems for guided missiles and in highaltitude aircraft installations.

Thus, in an aircraft altimeter of the type disclosed in .the copendingFischer application, Serial No. 807,782, filed April 2l, l959, nowPatent No. 3,068,699, there is entailed .an amplifier for controlling aservo motor. Such altimeters must work reliably within a temperaturerange of 55 to -{-l25 C., to meet stringent aircraft requirements. Themrplier must also be able to withstand the mechanical forces encounteredin aircraft operation.

Accordingly, it is the principal object of this invention to provide anamplifier which opera es efficiently and reliably even under adverseenvironment conditions.

Also an object of the invention is to provide an amplier of extremelycompact and light weight design and which adords maximum power andweight per unit volume.

Yet another object `of the invention is to provide a transistorizedamplifier of high input impedance, high voltage gain and high powergain, the performance of the circuit being maintained under difficultenvironmental conditions.

A further object of the invention is to provide an amplilier which is socontrolled as to atiord maximum A.C. and D.C. stability in thetemperature range of operation.

For a better understanding of the invention as well as other objects andfurther features thereof, reference is had to the following detaileddescription to be read in conjunction with the accompanying drawingwhose single FIGURE schematically illustrates a preferred embodiment ofthe amplifier circuit.

Referring now to the drawing, the amplifier, in accordance with theinvention, comprises a rst pre-amplifier stage including .a transistorlll, a second pre-ampliier stage including a transistor il, Ia driverstage including a transistor l2, and a push-pull output stage includinga pair or" transistors 13 and ld. ln practice, the transistors may Iallbe standard silicon devices, transistors lll and il being fof the 2N335type, transistor 12 of the 2N656- type and transistors 13 and i4 of the2N389 type.

An input signal introduced at terminals l5 is applied between the baseand emitter of transistor through capacitor 16. Hhe output of the firstpre-amplifier stage is taken from the collector electrode of transistorl@ and is applied through capacitor l? to the base of transistor lllrelative to the emitter thereof. The output of the second pre-amplifierstage is taken from the collector iof transistor il and -is appliedthrough capacitor 18 to the hase of the driver transistor 12.

The output of the driver transistor 12 is applied to the primary ot apush-pull input transformer 19, the ends of the secondary winding beingconnected to the respective bases of transistors if und i4. The centertap of the secondary of transformer i9 is connected through seriesresistors Ztl and 2l of the emitters of the transistors. The output ofthe push-pull transistors is applied to a load which, by way of example,is shown las the seriesconnected windings 22a and 2217 of a servo motor22, the junction of the windings being connected to the center tap ofthe secondary.

The amplifier is energized from 'a suitable D.C. Voltage source 23, suchas a 28 volt battery, the negative terminal being grounded and thepositive terminal going to the collector-base circuits of thetransistors in the several stages. Positive potential is applied to thecollector of transistor l0 through resistor 25, to the collector oftransistor ll through resistor 26, to the collector of transistor l2:through the primary of transformer 19 and to the collectors oftransistors 513 and 14- through the espective windings 22a 'and 22b ofservo motor 22.

in operation, a signal applied to input terminal 15 will be amplified inthe various stages of the amplifier to generate an energizing current inthe servo motor whose amplitude and phase will determine the direction'and extent `of movement of the servo motor. The input signal, in thecontext of the altimeter mentioned previously, acts as an error signalto cause the servo motor to move `a control element to a null position,at which point the signal is reduced to zero.

Since in practice the input signal is of low level and must preciselycontrol a servo system under circumstances where space is at a premiumand reliability is vitally important, it is essential that the amplifierbe highly efficient and stable. Accordingly, each stage of the circuitis designed for minimum variation of ygain :and .collector current andoverall negative A.C. feedback is used for AC. ygain stability.

Maximum DC. and A.C. stability are achieved by utilizing DJG. and A.C.feedback in each individual stage. The output of the first pre-amplifierstage is developed across resistor 25 in the collector circuit and aportion thereof is fed back negatively through resistor 27 to the basecircuit. Similarly, in the second stage, a portion of the output voltagedeveloped across resistor 26 is fed hack degeneratively through resistor2S. l

Resistors 215 .and 26 yare load resistors (RL) and resistors 2.7 and 28are base circuit feedback elements (RB). The combination of RB `and RLprovides both A.C. and D.C. feedback, only two components 'being used ineach pre-amplifier stage.

lt can be shown that:

A1, EQ2-[1 Tetu (l) a RB wherein lc=collector current ICO-:temperature-sensitive current a=common base current gain of transistorY 3 whereinz` R1==the ohmic value of resistor 2d R2=the ohmic value ofresistor 21H-1K2 secondary transformer and Ic, Ico represents the samequantities as in Equation 1. i

Overall A.C. gain stability is real-ized by utilizing a feedback networkbetween the output tof the final stage and the input of the first stage.To this end, the output voltage` i-s developed across the resistors 29and 30 connected in series between the collector of transistor 14 andground, a portion of the voltage `developed thereacross being fed to theinput of the first stage through resistor 31 connected between thejunction of the series resistors 29 .and 30 and the base circuit oftransistor 10. Resistors 29, 30 and 31 are preferably of precision type.It has been found that a [3K product 10 stabilizes the amplifier gain toWithin i3 db in the temperature range of operation.

It will be seen from the foregoing that the amplifier involves theminimum number of components to obtain high efficiency and stability.Thus it lends itself to miniaturizatifon. In :one practical embodimentof the invention, the circuit parameters areas given lin the drawing.MaXimum power .and weight per unit volume is obtained (2.1 watts percubic inch; 1.5 watts per ounce)high input impedance (50 kilo ohms)-highvoltage gain (60 db) and high power gain (85 dib).

It will be noted that the high input impedance at the inputterminal-s isdetermined essentially by the 50,000 ohm resistor interposed between theinput terminals 4and the base of the pre-amplifier transistor 10. Theshunt negative feedback paths in the pre-amplier circuit actVeffectively to decrease Ithe input impedance thereto to a very low valueapproaching zero. However, the input impedance of the transistor is inseries With the resistor, hence variations in this input impedance havea negligible effect on the input impedance as seen at the inputterminals 15.

While there has been shown what is considered to be a preferredembodiment of the invention, it will be maniest that many changes andmodifications may be made therein without departing from the essentialspirit of the invention. It is intended, therefore, in the annexedclaims to cover all such changes and modifications as fall within thetrue scope ot the invention.

What is claimed is:

1. A multiple-stage transistorized amplifier comprising at least onepre-amplifier stage each having a single transistor, means to apply aninput signal to said rst preamplifier stage through a relatively highxed impedance, a driver stage ycoupled to said pre-amplifier stage tandincluding `a single transistor and gain control means there for, ,and atinal stage coupled to said driver stage and including a pair oftransistors in pushapull relation, a :direct negative feedback path insaid preamplifier stage to stabilize same for both alternating anddirect current signals, a direct negative feedback path in said outputstage to stabilize same for direct-current, and a negative feedbacknetwork between said output stage and said input stage to provideoverall alternating current gain stability.

Y transistor, means to apply an input signal to said rst preamplifierstage through a relatively high fixed impedance, a driver stage coupledto the output of said second pre-amplier stage, and a final stagecoupled to said driver stage and including a pair `of transistors inpushpull relation, a Idirect negative feedback path in each of saidpre-amplifier stages to stabilize same for both alternating Iand directcurrent signals, -a direct negative feedback path in said output stageto stabilize same for directcurrent, and a negative feedback networkbetween said iinal stage and said first stage to provide overallalternating current gain stability, said network including a loadresistor connected to `one lof thetransistors in said final stage, and afeedback resistor connected capacitively between a point -in said loadresistor and the base circuit of said first stage.

3. A multiple-stage transistorized amplifier lcomprising first andsecond preamplifier stages each including a transistor, means to applyan input signal to said first preamplifier stage through a relativelyhigh fixed impedance, a driver stage coupled to the output Iof saidsecond preamplifier stage, :and a final stage coupled to said driverstage and including a pair of transistors connected in push-pullrelation, a direct negative feedback path in each of said pre-amplifierstages to stabilize same for both alternating and direct currentsignals, said path including a resistor connected between the ycollectorand base of the transistors in each pre-amplifier stage, a Idirectnegative feedback path in `said iinal stage to stabilize same fordirect-current, and a negative feedback network between said outputstage and said input stage to provide overall alternating current gainstability, said network including a load resistor connected to onetransistor in said final stage and Aa feedback resistor connectedcapacitively between a point in said load resistor and the base circuitof said first pre-amplifier.

References Cited in the tile of this patent UNITED STATES PATENTS2,762,873 Goodrich Sept. 11, 1956 2,784,262 Crow Mar. 5, 1957 2,852,625Nuut Sept. 16, 1958 2,951,991 Rickner a Sept. 6, 1960 2,989,628 Hergantime 2o, 1961 3,008,092 Cawthara Nov. 7, 1961 FOREIGN PATENTS Y 757,214Great Britain Sept. 19, 1956 OTHER REFERENCES Herscher: DesigningTransistor'A-F Power Amplifiers, Electronics Engineering Edition, April1l, 1958, pages 96-99.

Dion: Common Emitter Transistor Amplifiers, Proc. IRE, May 1958, page920.

Aronson: Transformerless Transistorized A-F Power Amplilier iHas 7-WOutpu Instruments and Automation, August 1958, page 1371.

Hinrichs et al.: Squarved Input Stages for Low-Level TransistorAmplifiers, 1958, IRE Wescon Convention Record, vol. 2, part 2, pages104-114 (page 111 only relied on)

1. A MULTIPLE-STAGE TRANSISTORIZED AMPLIFIER COMPRISING AT LEAST ONE PRE-AMPLIFIER STAGE EACH HAVING A SINGLE TRANSISTOR, MEANS TO APPLY AN INPUT SIGNAL TO SAID FIRST PREAMPLIFIER STAGE THROUGH A RELATIVELY HIGH FIXED IMPEDANCE A DRIVER STAGE COUPLED TO SAID PRE-AMPLIFIER STAGE AND INCLUDING A SINGLE TRANSISTOR AND GAIN CONTROL MEANS THEREFOR, AND A FINAL STAGE COUPLED TO SAID DRIVER STAGE AND INCLUDING A PAIR OF TRANSISTORS IN PUSH-PULL RELATION, A DIRECT NEGATIVE FEEDBACK PATH IN SAID PRE-AMPLIFIER STAGE TO STABILIZE SAME FOR BOTH ALTERNATING AND DIRECT CURRENT SIGNALS, A DIRECT NEGATIVE FEEDBACK PATH IN SAID OUTPUT STAGE TO STABILIZE SAME FOR DIRECT-CURRENT, AND A NEGATIVE FEEDBACK NETWORK BETWEEN SAID OUTPUT STAGE AND SAID INPUT STAGE TO PROVIDE OVERALL ALTERNATING CURRENT GAIN STABILITY. 